Powder coating systems are systems in which coating is deposited upon a substrate or article in the form of a powder and the deposited powder is then heated so that it will flow and harden on the surface of the substrate or article being coated. It is generally conventional in such systems to inject an electrostatically charged airborne fluidized powder into an enclosure or booth containing the product to be coated with an electrostatic potential applied between the powder and the object to electrostatically attract the powder onto the substrate surface.
In such a system, the pressurized "shop air" from the facility is typically passed through an air dryer which reduces its dew point, initially at least, to, for example, approximately 38.degree. F. at the shop air pressure. This partially dried supply air is then fed from the dryer through an air supply line and through various solenoid valves which are operated to control the distribution of air to various parts of the powder coating system. For example, the supply air is used to fluidize powder from a bulk powder supply, which ultimately is transported to the powder spray booth. Supply air is also used to pump the fluidized powder to a cyclone and sieve which removes air from the powder, cleans the powder and drops it into a feed hopper. Supply air is also used to pump the fluidized powder from the feed hopper to a spray gun. In addition, the control of the air-to-powder ratio supplied to the gun is achieved by air supplied through a pair of air lines as the fluidized mix is transferred out of the feed hopper.
Additionally, air is provided to a transfer pump mounted on a recycle hopper which is located at the bottom of the spray booth to transfer oversprayed powder, which does not adhere to the object being coated, out of the recycle hopper. The transfer pump transports the powder collected in the recycle hopper to the cyclone and sieve for cleaning and redeposition into the feed hopper so that it can be recycled to the spray gun.
Powder sprayed from the gun which does not adhere to the product, referred to as the oversprayed powder, is removed from the air in the powder booth and deposited into the recycle hopper by a bank of cartridge filters which are periodically pulsed by high pressure air valves controlled by a timer device. During pulsing, high pressure air is blown in the reverse direction into the cartridges to knock the powder off the outside of the cartridges which causes the powder to fall into the recycle hopper so that it can be returned to the feed hopper for reuse.
In powder coating systems, it is possible for the dew point of the supply air to rise to the point where water can condense out of the air being utilized in the system. Condensation in the various air lines of the system can cause a poor quality finish on the product and clogging of the cartridge filters.
Prior art systems have been ineffective in preventing such condensation or in providing adequate advance detection of the conditions under which such condensation is likely to occur. Consequently, there is a need for a system for detecting the occurrence of condensation in a powder coating system or the conditions under which condensation is likely to result.
Furthermore, control of the powder coating systems has been traditionally carried out through the use of various separate control devices having digital and analog inputs and outputs which connect with various valves, servos, sensors and switches of the system to control, in a semiautomatic manner, the operation of the system. This prior art practice requires custom engineering and special configuration of each control system to conform to the characteristics of the particular system, and accordingly, design of the controls for such systems have been necessarily specific to the systems in which they are employed. Consequently, the use of sophisticated controllers has been confined principally to the larger scale systems.
Therefore, there is also a need to provide flexible controllers for use in diverse powder coating systems which do not require custom design.